dirstate: Use the Rust implementation of DirstateItem when Rust is enabled
… instead of the C implementation, with C/Rust conversions at the FFI boundary
Differential Revision: https://phab.mercurial-scm.org/D11486
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed